diff --git a/bin/d3/dune b/bin/d3/dune
new file mode 100644
index 0000000..41343f8
--- /dev/null
+++ b/bin/d3/dune
@@ -0,0 +1,4 @@
+(executable
+ (public_name d3)
+ (name main)
+ (libraries core utils))
diff --git a/bin/d3/input.txt b/bin/d3/input.txt
new file mode 100644
index 0000000..0e4ee5e
--- /dev/null
+++ b/bin/d3/input.txt
@@ -0,0 +1,300 @@
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diff --git a/bin/d3/main.ml b/bin/d3/main.ml
new file mode 100644
index 0000000..de6fd30
--- /dev/null
+++ b/bin/d3/main.ml
@@ -0,0 +1,47 @@
+let split s =
+    let half = (String.length s) / 2 in
+    (String.sub s 0 half, String.sub s half half)
+
+let find_duplicates s1 s2 =
+    String.to_seq s1
+    |> Seq.filter (fun c -> String.contains s2 c)
+    |> Seq.fold_left
+        (fun acc c -> if String.contains acc c then acc else acc ^ String.make 1 c)
+        ""
+
+let priority item =
+    let num = int_of_char item in
+    if num > 96 then num - 96
+    else num - 38
+
+let find_duplicates3 s1 s2 s3 =
+    find_duplicates s1 (find_duplicates s2 s3)
+
+let get_duplicate dup =
+    try Some(String.get dup 0 |> priority)
+    with Failure _ -> None
+
+open Utils
+
+let part1 l = l
+    |> List.filter_map (fun s ->
+        split s
+        |> uncurry find_duplicates
+        |> get_duplicate)
+    |> List.fold_left (+) 0 ;;
+
+let triplets l =
+    let rec aux res = function
+        | [] | [_] | _ :: _ :: []-> res
+        | elv1 :: elv2 :: elv3 :: tail -> aux ((elv1, elv2, elv3) :: res) tail in
+    aux [] l ;;
+
+let part2 l = l
+    |> triplets
+    |> List.filter_map (fun triplet ->
+        (uncurry3 find_duplicates3) triplet
+        |> get_duplicate)
+    |> List.fold_left (+) 0 ;;
+
+"\n1: " ^ string_of_int (read_file "bin/d3/input.txt" |> part1) ^
+"\n2: " ^ string_of_int (read_file "bin/d3/input.txt" |> part2) |> print_endline
diff --git a/lib/utils.ml b/lib/utils.ml
index 0f1ada6..08d22ad 100644
--- a/lib/utils.ml
+++ b/lib/utils.ml
@@ -9,3 +9,18 @@ let read_file name =
         | None -> close_in ic; acc in
     aux []
 
+module CS = Set.Make(Char);;
+
+let set_of_string str =
+    let rec aux s i st =
+        if i < 0 then st
+        else aux s (i - 1) (CS.add s.[i] st) in
+    aux str ((String.length str) - 1) CS.empty
+
+let map_pair f (a, b) = (f a, f b)
+
+let curry f x y = f (x, y)
+let uncurry f (x, y) = f x y
+let uncurry3 f (x, y, z) = f x y z
+
+let swap x y = y x